Path: utzoo!mnetor!uunet!husc6!necntc!ima!haddock!karl
From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: global data in C programming...
Message-ID: <3032@haddock.ISC.COM>
Date: 17 Mar 88 17:18:54 GMT
References: <7506@ncoast.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 36
Keywords: global data, extern, C programming
Summary: declare it N times, but define it just once

In article <7506@ncoast.UUCP> btb@ncoast.UUCP (Brad Banko) writes:
>i thought that you just create a header file containing the global data
>declarations and then #include it into each of your source files, but
>when i try this (using Mark Williams C on an Atari ST), I get
>a "symbol redefined" message from the linker.

Yes.  After the preprocessing phase is through, what you have is a set of
source files each of which contains "int i, j;".  Although this is accepted by
some implementations, it is not portable.

"extern int i, j;" is a declaration; "int i, j;" is a definition.  Each object
must have exactly one definition.  The best$ way to do this is to change your
header file to use the "extern" keyword, and pick one source file (or make a
new one) to contain the definitions.  Thus:

x.h:
	extern int i, j;

x.c:
	#include "x.h"
	xyz() { ... i = ... }

extern.c:
	#include "x.h"
	int i, j;

If you want them to be initialized to non-zero values, put the initializers in
extern.c ("int i=3, j=4;").  (Actually, some implementations seem to require
the initializers anyway, to distinguish a definition from a declaration; so
you might want to add them even if they're zero.)

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
________
$ Another popular style, which I do not recommend, is to have x.h contain
"global int i, j;" where "global" is a macro defined with a null expansion in
extern.c but expanding to "extern" for everyone else.